Secrecy communication system



SECRECY COMMUNICATION SYSTEM Jane n, 19

CRAIB sheets-sheet 2v Filed .April` 29, 1942 IW o.

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m'mooooooommmmm m MOE NOE INVENTOR JAMES F. CRAIB BY QW ATT RNEY N). 4+0.0 OO

IMT oO June 11, 1946- J. F. cRAlB 2,402,059

SECRECYQOMMUNICATION SYSTEM Filed April 29, 1942 s sheets-sheet s JAMESF. GRA|B Patented June ll, 1946 carton srs'rniu Application April 29,1942, Serial No. 441,024

(Ci. Mii- 1.5)

3l Claims. l

The present invention relates to secrecy cornmunication systems and,particularly, to such systems which are especially suitable for thetransmission of intelligence in the form of speech signals. In greaterparticularity, the invention relates to secrecy communication systems ofthe type in which frequency-component subbands of a speech signal areperiodically modified, as by sluiting, inverting and interchanging thesubbands in the frequency spectrum, thus tp modify the intelligibilityof the signal when reproduced in conventional manner.

Many systems have heretofore been proposed by which to effect, to agreater or lesser degree, secrecy of communication of speech signals,especially for radio communications which are easily intercepted byunauthorized persons. A secrecy communication system is consideredsatisfactory from a practical standpoint only when intelligiblereproduction of communications by an unauthorized person is practicallyimpossible, even though such person has a complete receiving apparatuscapable of rendering the received signals intelligible and lacks onlyinformation concerning the preestablished procedure by which thetransmission is rendered unintelligible.

t is desirable that a secrecy communication system interrupt andotherwise modify the communication to be transmitted at such a rapidrate as to render the communication so unintelligible that anunauthorized person receiving the communication is unable to understandeven syllables of the speech'signal .when reproduced in conventionalmanner, or when reproduced merely with frequency-component subbands ofthe received signal displaced in the frequency spectrum since, wherethis is possible, the speech signal is' in general reproducible asintelligible speech portions against a background of severe noise and askilled person is frequently able to understand the communicationFurther, it is desirable that the rapidity of interruption andmodification of the speech signal be so high that intelligibility of thecommunication cannot be restored without f great expenditure of time andeffort when'accoin plished by the method of recording the communicationon both sound and visual records, analyzing the visual record todetermine the preestablished plan of procedure by which the transmissionis rendered unintelligible, and. using this information, reproducingthe'recorded sound record with completely restored intelligibility ofcommunication. Lastly, it is desirable that the apparatus necessary toeffect secrecy of 'communication be relatively light in weight, compact.

inexpensive, and adapted satisfactorily to operate Without anyadjustments other than the initial adjustments usually made by themanufacturer of the apparatus; that the plan, of procedure of renderingthe communication unintelligible and of restoring intelligibiiity beeasily, quickly and readily accomplished; and that the band widthrequired to translate the unintelligible signal should not appreciabiyexceed the band width required to translate the speech ,signal before itis rendered unintelligible.

It is an object of the present invention, therefore, to provide a newand improved secrecy communication system of the type described and onecharacterized by a higher degree of secrecy of communication than hasheretofore been at- A tainable by known methods and apparatus.

it is a further object of the invention to provide a secrecycommunication system particularly suitable for the transmission ofspeech signais and one in which one or more groups or subbands of thefrequency components .of the speech signal are interchanged relative toeach other, shifted or inverted in the frequency specgtrurn, orinterchanged, shifted and inverted in a predetermined order at intervalsnot substantially longer than the longest syllabic interval of thespeech signal.

It is an additional object of the invention to provide a new andimproved secrecy communication system in which the signal to becommunicated is rendered unintelligible by modifying the frequencycomponents thereof in accordance with any selected one of a very largenumber of preestablished plans `of procedure which are adapted to bereadily and quickly changed from time to time, as desired.

It is an additional object of the invention to provide a new andimproved secrecy communication system having any or all of the featuresspecified above as desirable in a system of this nature. f

In accordance with one embodiment of theinvention, e. secrecycommunication system comprises an input circuit adapted to have appliedthereto a speech-signal to be transmitted, the signal comprisingfrequency components extending over a predetermined frequency band, Thesystem includes meansA for deriving from the signal a second signalcomprising a plurality of frequency-component subbands corresponding toindividual` portions oi the aforesaid frequency band, thefrequency-component subbands of the second signal being successivelymodified relative to each other in a predetermined order at intervalsnot substantially longer than the longest syllabic interval of thespeech signal. The system also includes means for translating andreceiving the second signal and for deriving therefrom the speechsignal.

In a particular form of the invention, a secrecy communication system ofthe type described -includes a plurality of band-pass filters forseparating the speech signal into a plurality of subbands correspondingto individual portions ,of the speech-signal frequency band, a pluraltiyof modulators individually coupled to the band- '\pass filters, meansfor deriving a plurality of displacement carrier waves, means, includingelectronic switching means, for successively applying the carrier wavesindividuallyY to themodulators in a predetermined order during intervalsnot substantially longer than-the longest syllable interval of thespeech signal to derive a second signal having modified intelligibility,and an output circuit coupled to the modulators to have applied theretothe second signal, whereby the signal applied to the output circuit issuitable for transmission therefrom to a remote point while maintaininga high degree of secrecy of communication.

, For a better understanding of the present invention, together withother and further objects thereof, reference is had to the followingdescription taken inconnection with the accompanying drawings, and itsscope will be pointed out in the appended claims. v

Referring now to the drawings, F18. 1 is a cir- 'cuit diagram, partlyschematic, of a complete carrier-wave transmitter suitable for use in asecrecy communication system embodying the in-4 vention; Fig. 2comprises graphs representing the manner in which a speechsignal may be`divided linto frequency-component subbands and the man- -gram of aportion of a secrecy communication 'system and represents a modifiedform ordis-- placement carrier-wave generator suitable for use in thetransmitter and receiver arrangements of Figs. 1 and 4.

Referring now more particularly to Fig. 1, there is represented,schematically, a complete carrierwave transmitter suitable for use in asecrecy communication system embodying the invention in a preferredform. The transmitter includes an input circuit, comprisinginput-circuit terminals I0, II, ladaptedto have applied thereto a speechsignal to be transmitted. This speech signal has somewhat modifiedintelligibility and is derived from a speech signal of fullintelligibility picked up by a microphone I2 and applied through anaudio-frequency amplifier I2, of one or more stages, to an input circuitof a modulator I4, the latter unit having another input circuit towhich, is coupled a displacement carrierwave generator I5. The speechsignal picked up by the microphone I2 has frequency components extendingover a predetermined frequency band,

.for example from 300 to 3000 cycles, and the speech signal to 'betransmitted and which is applied to the input-circuit terminals I0.. II, has

For example, frequency components of 300 and 3000 cycles in the speechsignal correspond to i the respective frequency components of 8700 and6000 cycles in the speech signal to be transmitted when the generator I5has a frequency of 9000 cycles. The transmitter includes means forseparating the speech signal to be transmitted into a plurality offrequency-component subbands corresponding to individual portions of thefrequency band of this signal. This means comprises a plurality ofband-pass filters I6, Il and I8 having their input circuits connected tothe output circuit of the modulator I4. Thus, assuming the speech signalto be transmitted has a frequency band from 6000 to 8700 cycles, theband-pass filter I8 may derive, for example, a frequency-componentsubband corresponding to the portion extending from 8700 to 7800 cyclesof the frequency band, the band-pass filter I1 may derive that portionfro'm 7800 to 6900 cycles, and the band-,pass filter I8 may derive theportion from 6900 to 6000 cycles.

There is also included in thetransmitter a plurality of modulators I8,20 and 2|, each having a signal input circuit, and these input circuitsbeing individually coupled to the output circuits of theband-pass'filters I6, I1 and I8. The output circuits of the modulatorsI8, 20 and 2l are coupled to the input circuit of a low-pass filter 22through respective amplifiers 22, 24 and 2l. The output circuit of thefilter 22, in turn,l is coupled to the input circuit of a carrier-wavetransmitting apparatus 28 which includes a source of carrier waves, amodulator, and one or more stages of radio-frequency amplification. Theoutput circuit of unit 28 is coupled to a radiating antenna system 21,28.

There is also provided in the transmitter means for deriving a pluralityof displacement carrier waves, this means comprising a plurality ofcarrier-wave generators 28 to 28, inclusive, having respectiveindividual frequencies f1 to fa, inclusive. The frequencies f1 to fa.inclusive, may havethe respective values. for example, of 10,800 cycles.9900 cycles, 7500 cycles, 6600 cycles, 8100 cycles, 4800 cycles, 7200cycles, and 3900 cycles. There is also included in the transmitter acarrier-wave repeater unit 1 comprising a plurality ofyrepeater devices31 to 48, inclusive, arranged in groups, lfor example 31 to 40,inclusive. 4I to 44, inclusive. and 45 to 48, inclusive. Each such grouphas a common output circuit which is coupled to a carrierwave inputcircuit of an individual one of the modulators I9, 20 and 2l. Each o fthe repeater devices 31 to 48, inclusive, has its input electrodescoupled to a given one of the generators 28 to 28, inclusive,` and is,in turn, effective to couple the generator. to which it is so coupled,to an associated one f the modulators I9, 20 and 2| when 'the repeaterdevice is suitably energized and has a suitable bias applied between itsinput electrodes. The repeater devices 31 to 48, inclusive, areenergized from sources of space current, indicated as +B,` but the inputelectrodes of each asoaoeo are normally biased below anode currentcutoff from sources of biasing potential indicated as -C. At any giveninstant during the operation of the transmitter of Fig. l, one repeaterdevice in each of the groups 31 to Q0, inclusive, il to im, inclusive.

" and Q5 to 68, inclusive, is biased above cuto by electronic switchingmeans 39, and the interval during which each tube is so biased is notsubstantially longer than the longest syllabic interval of the speechsignal. The term syllabic interval of speech will hereinafter bediscussed in greater detail and defined during the description of theoperation of the invention. Unit 69 has a plurality of control circuitssuccessively energized at intervals not substantially longer than thelongest syllable interval of the speechsignal. each such circuit beingcoupled to the one of the aforesaid repeater devices of thebefore-mentioned groups through manually-adjustable switching means 513.The manually-adjustable switching means 5B is provided so that the orderof coupling of each or' the control circuits of unit te to the repeaterdevices 31 to i, inclusive, can be established and modied, as desired,from time to time.

The electronic switching means i9 comprises a plurality of identicalswitching units iii to inclusive, only the ilrst of which is shown indetailed schematic form. Each oi the switching units 5i to 553,inclusive, includes an input circuit, comprising input-circuit terminals59, 59', and an output circuit comprising output-circuit terminals d,Si. Each of these units also includes means responsive to a pulse ofnegative potential applied to the input circuit thereof for developingin the output circuit thereof a pulse of positive potential. This meanscomprises a pair of vacuum tubes 62, S3, the input electrodes of tube e2being coupled to the input-circuit terminals 59, 59' and the outputelectrodes o f tube 63 being coupled to the output-circuit terminals tu,6l. The lastnamed means also comprises means intercoupling the input andoutput electrodes of the tubes 62. 63

to develop the positive-potential pulse in the output circuit of theunit in response to the application of a negative-potential pulse to theinput circuit thereof. The intercoupling means of each of units 5l to59, inclusive, comprises a voltage divider, comprisingserially-connected resistors Gil,

65, which couples the output circuit of tube-52 to y the input circuitof tube 53 and a time-constant circuit, comprising a condenser vlit andan adjustable grid resistor 61, which couples the output circuit of tube53 to the input circuit of tube 62. The control circuit of unit 5l,which comprises control-circuit terminals 6i land SB, is coupled to theoutput circuit of tube 63. The electronic switch also includes means forcoupling the .units 5i to 53. inclusive, in cascade., In order to effectthis connection, the output circuit of eachunit is coupled to the inputcircuit oi' a succeeding unit, the coupling means comprising, in eachcase, a condenser te.

There is also included in the electronic switch fle, for a purposepresently to be considered, an additional switching unit 1t comprisingan input circuit including input-circuit terminals ll, 1i. an outputcircuit including output-circuit terminals 12, 13, and means responsiveto a negative-potential pulse appliedto the input circuit for developinga negative-potential pulse in the output circuit. The last-named meanscomprises a pair of vacuum tubes 1d, 15, the output circuit of tube lbeing coupled to the input circuit of tube it by a voltage divider,cmprising series-con- 6 nected resistors 1t, 11, and the output circuitof tube 15 being coupled to the input circuit of tube 14 by atime-constant circuit comprising a condenser 18 and an adjustable gridresistor 19. The

input electrodes of tube 14 are coupled to the input-circuit terminals1|, 1|', whilethe output electrodes of tube 15 are coupled to theoutputcircuit terminals 12, 13 and the latter terminals are coupled, inturn, to the input-circuit terminals 59, 59' of unit 5I by means of acoupling condenser 68. Thus, units 10 and 5| are similar in circuitarrangement, except that the outputcircuit terminals 12, 13 of theformer are coupled across the output electrodes of tube 15, whereas theoutput-circuit terminals eil, 6l of unit 5! are coupled to the cathodecircuit impedance of tube 153. The cascade connections of the units ofthe electronic switch 39 are completed by coupling the output circuit ofthe last cascaded unit 5t to the input circuit oi the additional unit l@through a coupling condenser 6e.

The manually-adjustable switch 5h includes a plurality of verticalconductive bars @il to t1, inclusive, which are connected to individualones o the control circuits of units 5l to t, inclusive, of theelectronic switch lli?. The unit t@ also includes a plurality oi.horizontal conductive bars et to et, inclusive, which are connected tothe control electrodes of individual ones of the repeater devices 31 toet, inclusive. The latter bars may be divided into groups I, l, and IIIof four bars each to designate, for convenience, the grouping of thebars ,with relation to the respective groups of repeater devices 3l tol, inclusive, il to titl, inclusive, and 65 to 453,.

inclusive. Manually-adjustable switch elements are included in unit ellso that the operator can electrically connect any one of the verticalbars t@ to u?, inclusive, to any one of the horizontal bars et to @9,inclusive. These switch elements may comprise a spring-biased contact,not shown' for simplicity, which normally would electrically connecteach vertical bar to every one of the horizontal bars at theircross-over pointsL However, only selected ones of the switch elementsare permitted to complete the electrical connection between the verticaland horizontal bars. This is accomplished by the insertion or a sheet ofinsulating material between the switch elements and the vertical orhorizontal bars which the switch elements would normally engage,appropriate holes vbeing punched in the sheet of mate- I rial to form acode card and to permit engagement of only selected vertical elementswith their cooperating horizontal bars. The engaging switch elements forone such code card are illustrated in Fig. i of the drawings by thesolid black circles, and it will be noted that each of the bars Bu to81, inclusive, is electrically connectedto one, and only one, horizontalbar in each of the three groups I, Il, and III.` Thus, the `:ontrolcircuit of each of units El to be, inclusive, is electrically connectedthrough the manually-ML justable switch tu to the control electrode cione repeater device in each of the three groups oi repeater devices 31to d, inclusive, il to lii, inclusive, and t5 to (it, inclusive.Preferably, the manually-adjustable switch E@ is constructed with thevertical bars secured on one support member and the horizontal barssecured on another supportmember, these members being hinged linbook-like fashion and one of them supporting the engaging switchelements,V a construction which permits -ready lnsertion of the codecard between the vertical and horizontal inafter designated as "speechsignal Y.

bars and the switch elements cooperating theref distinguish it from thespeech signals of modified intelligibility derived therefrom and havingfrequency components corresponding thereto. The speech signal X isamplified-by the audiofrequency amplifier I3, and applied to one inputcircuit of the modulator I4. There is applied to another input circuitof the modulator Il a speech-displacement carrier wave generated by thegenerator I5. There is thus produced in the output circuit of modulatorIl a modulated car- 'rier wave having modulation sidebands, each ofwhich contains frequencv components corresponding to the speech signalX. However, these modulationsidebands are each displaced upward in thefrequency spectrum from the speech signal X and the frequency componentsof the lower sideband are inverted with respect to the correspondingfrequency components of the speech signal X. Since suchfrequency--component inversion is useful as a first step Ain thedirection of modification of the intelligibility of the speech signal,the lower sideband alone-is selected and utilized in the apparatus whichfollows the modulator I4 and such sideband thus comprises another speechsignal to 'be transmitted and is here- The purpose in using thedisplacementcarrier generator I5 to derive the speech signal Y isprimarily to shift the frequency components of the 4speech signal X to`a higher portion of the frequency spectrum where they may be morereadilyhandled for purposes of interchanging. shifting, and invertingthem in the frequency spectrum ultimately to derive the unintelligiblespeech signal to be transmitted.

. Assuming, as before, that the displacement carrier generator I5 has afrequency' of 9 kilocycles and the speech signal a frequency band from300 to 3,000 cycles, the modulated displacement carrier wave applied tothe input-circuit terminais I0, II of the transmitter has 'a lowersideband extending from 8.7 to 6 kilocycles, comprising the speech'signal Y as hereinbefore stated,

` and an upper sideband extending from 9.3 to 1,2

kilocycles which is not translated. As'previousiy stated, the band-passfilters I6, I1 and I8 translate only the respective pass bands extendingfrom 8.7 to 7.8 kilocycles, 7.8 to 6.9 kilocycles, and 6.9

- to 6.0 kilocycles. Consequently, the band-pass filters I8, I'I and I8separate-the speech signal Y into a -plurality of frequency-componentsubbands corresponding to `individual portions of the frequency band ofthis signal. It may be noted that these subbands correspond to similarsubbands into which the speech signal X might be divided. For example,the subbands (3.7-7.8 kilocycles, 7.8-6.9 kilocycles, and 6.9-6.0kilocycles of the speech signal Y correspond in information content ofthe signal to be transmitted to the respective subbands .3-l.2kilocycles, 1.2-2.1 kilocycles, and 2.1-3.0 kilocycles of the speechsignal X. The subbands derived by the filters ,|6, I1 and I8 areindividually applied to the Vsignal input circuits of the respectivemodulators I8, 20 and 2|.

Referring now to Fig. 2 of the drawings, the frequency-componentsubbands derived by the band-pass filters I6, I1 and I8 from the speechsignal4 Y are designated in Fig. 2 as A, B and C,

8 respectively. It will be noted that each of these subbands is 0.9kilocycle wide. Further to modify the intelligibility of the speechsignal .Y, the frequency-component subbands A, B and C are successivelymodifiedrelative to each other in a predetermined order at intervals notsubstantially longer than the longest syllabic interval of the speechsignal X. This is accomplished by apparatus, presently to be considered,which operatesy to interchange the position of the subbands, invert thefrequency components of individual ones of the subbands, or bothinterchange the position of the subbands and also invert the frequencycomponents of one or more thereof. Actually, as

will presently become apparent, the relative shift... .ing of thesubbands A to F, inclusive, does not occurin the frequency band 8.7-6.0kilocycles, but

takes place effectively as though the subbands had first been 'shiftedwith relation to each other in this frequency band to derive a signal ofmodified intelligibility, which may be designated as sigvnal Z, and thatthereafter the signal Z had been so shifted in the frequency spectrumthat its frequency components occupied the frequency band of .3-3.0kilocycles. -However, for purposes of simplicity in explaining themanner oflforming thequency spectrum of 8.7-6.0 kilocycles, the invertedsubbands .corresponding to the subbands A, B and C then would occupy theposition indicated in Fig. 2 as D, E and F, respectively, in which thesubbands appear in their original order. It will be evident, of course,that the ultimate speech signal to be derived, -that is, the speechsignal having the highest degree of unintelligibility and which is to betransmitted, must include frequency components corresponding to theentire number of frequencycomponents of the speech signal X. This may beeffected, for example, by adding the subbands A and C to the invertedsubband E or by adding the inverted subbands D and F to the subband B.When the subbands A-F, inclusive, are thus added in all possiblearrangements containing the complete original speechfrequencycomponents, it can be shown that it i is possible to provide 64arrangements of the subbandswhile yet transmitting frequency componentscorresponding to al1 frequency components of the speech signal X.

However, the transmission of subbands A, B and C in their originalposition in the frequency band of '8.7-6.0 kilocycles would beintelligible when reproduced by an unauthorized listener if he simplyinverted the frequency components of these subbands andshifted them to.the corresponding frequency rangel of .3-3.0 kilocycles of the originalspeechsignal X. This also applied to the inverted subbands D, E and Fwhen used in their origina1 position, except that these subbands needmerelyhe shifted to their corresponding position in the original speechsignal X, the inversion step having already been performed on them.

Hence. a higher degree of secrecy may be effected by forming theultimate signal of modified aeoaose 9 intelligibility which is to betransmitted only of frequency-component subbands in which nofrequency-component subband, or inverted subband, of the speech signal Yoccurs in its normal position with relation to the other two subbands.

This reduces the number of possible arrangements of thefrequency-component subbands which may beused to form the ultimatesignal of modified intelligibility to sixteen. These are tabulated inFig. 3. Considering specically one such arrangement of subbands, whichis designated in Fig. 3 las code number I, it will be noted that thesub- ,Y band B has been shifted into that portion of the frequencyspectrum normally occupied by-subband A, that subband A is shifted tothe position normally occupied by subband B, these two subbands thusbeing merely interchanged, and that subband C has effectively beeninverted and shifted in the frequency spectrum to the position which itnormally occupies without inversion.

kBefore considering the precise manner in which the frequency-componentsubbands are shifted. interchanged relative to each other, inverted, andadded to derive the ultimate signal of modified intelligibility, thedetailedoperation of the electronic switch de will rst be considered.

In this discussion, frequent reference will be made to the longestsyllabic interval of the speech signal and it is,I therefore, deemedadvisable at this point more fully to deiine the meaning of this term;It -has been observed that ordinary speech is eilected by. Wordsyllables and that the lowest frequency of occurrence of such syllablesaverages approximately twenty times per second. On this basis, thelongest syllabic interval is 50 milliseconds. This, of course, is anaverage value and in isolated cases the syllabic interval may increasesubstantially to that of the x word interval which, in ordinary speech,has been found to have a value of approximately 70D-800 milliseconds asdetermined byaverage speech of approximately one-hundred words perminute.- Hence, in the present specification and claims, the termlongest syllabic interval" of the speech signal may be defined as aninterval of from approximately 50 milliseconds to 700 or 800milliseconds.

Considering now in detail the operation of the electronic switch t9, theiirst tube in each of the units 5l to E8, inclusive, and 79, for exampletube 52 of unit 5| 'and tube ri of unit lt, is normally conductive andthus has relatively high conductance, and the second tube of each ofthese units is normally nonconductive and hence has low conductance.

Now assume that a negative-potential pulse is applied to theinput-circuit terminals 'ii and li of unit 7d. This pulse causes tube leto become nonconductive and applies a positive pulse to tube 'i5 torender the iatr conductive. When tube 'i5 becomes conductive, a negativepulse is appliedto the input-circuit terminals 5t, Se' of unit el, thuscausing tube 62 of this unitto be nonconductive, thereby to apply apositive pulse to the control electrode of tube 63. There is coupledfrom the output circuit of unit El to the input circuit of unit 52 apositive pulse at the time that tube ed becomes conductive, but thispulse has no eiect on the operation of unit 52, since the iirst tube ofthat unit is already in a conductive state.- At the same time, anegative pulse is applied from the output circuit of tube S3 through thecondenser Se and the resistor 61 to the control electrode of the lirsttube 62 of unit lfand this pulse has a duration, determined by anegative pulse is applied from the output circuit f 62 to the controlelectrode of tube 63 to reduce the conductivity of the latter, theaction being rapid and cumulative to render tube` 62 fully conductiveand tube 63 nonconductive. .At this time,

of unit 5l to the input circuit'of unit E2, thus to render the firsttube of the latter unit nonconductive and the second tube of such unitconductive for an interval of approximately 50 milliseconds, as in unit5 I.

Consequently, at the end of each such interval, a succeeding oneA of theunits 53 to 58, inclusive, is caused to produce a. positive potentialpulse in its output circuit. The last unit 58 is arranged to apply anegative pulse to the input circuit of I unit 10 at the end of the 50millisecond interval of operation of the former unit. During the timerequired for the units El to 58, inclusive, to become successivelyoperative, the first tube It of unit 'l0 has been maintainednonconductive by a negative pulse of longer duration, determined by thetime-constant of elements i8 and 79, which is applied from the outputcircuit of tube 75 to'r the control electrode of tube 1li. Tube labecomes fully conductive, however, at a time just preceding the time atwhich the negative potential pulse is appliedy from unity 5% through thecoupling condenser B9' to the input circuit of unit 1li. .Consequently,unit l@ may be considered a barrier unit in that it is not affected byundesired spurious potential pulses applied to its input circuit duringthe major portion of the interval between the application thereto ofdesired negative pulses from the output circuit of unit 58. It will thusbe evident that the plurality of control circuits of unit da aresuccessively'energized at vintervals not substantially longer than thelong- 32 to the modulator it, from the generator tt to y the modulator2b, and from the generator 3d to the modulator 2 l. Similarly, when thenext vertical bar Sil has a positive potential pulse applied theretofrom unit t9, therepeater devices 38, d2 and it are conditioned to applycarrier-wave oscillations from the respective generators 30,' 33 and 3Gto the respective modulators I9, 2@ and 2i.

The carrier waves applied to the modulators i9. 2li and 2l are modulatedtherein by the frequency-component subbands applied to the modulatorsfrom the filters iii, il and i8; There is thus developed in the outputcircuit of each modulator a carrier wave having upper and lowermodulation sidebands. As will presently become apparent, only the lowermodulation sideband of these modulated carrier waves is' used in formingthe speech signal of modied intelliglblllty signal of modifiedintelligibility may readily be translated through conventionalaudio-frequency translating amplifiers and circuits, but also that itrequires minimum band width for the modulated carrier Wave which isradiated by the antenna system 21, 28. Assuming that the signal ofmodified intelligibility is to occupy a frequency band ofthe same width,and to have the same mean frequency, asthat of the speech signal derivedby the microphone I2, for example 300- 3,000 cycles, the frequency thateach of the generators 29 to 36, inclusive, should have is governed bytwo considerations. First, the lower sideband of the modulated carrierWave developed in the output circuit of each of the modulators I9, and2| should always lie within the frequency range from G-3,000 cycles;and, secondly. the frequency components of the lower modulation sidebandshould correspond tothe frequency components of a corresponding one ofthe subbands A, B and C, or inverted subbands D, E and F, Fig. 2, whenthe latter subbands are 4 shifted in the frequency spectrum relative toone or more such subbands. To illustrate this by way of example,consider what occurs when the vertical bar 8D of switch 50 is energizedwith a `positive-potential pulse fromI the lelectronic switch 49. It haspreviously been shown that this is effective to couple generators 32, 33and 36 through the respective repeater devices 40, 42 and 48 to therespective modulators I9, 20 and 2|. The lower sideband of the.modulated carrier wave developed in the output circuit of modulator I9includes frequency components in the band of 2.1-1.2 kilocycles in-lwhich the frequencies 2.1 and 1.2 kilocycles correspond, respectively,to the frequencies 8.7 and 7.8 kilocycles of the subband A of speechsignal Y. Now if the entire speech signal Y were to be shifted into thefrequency bandof .3-3.0 kilocycles, as by the use of a 5.7 kilocyclecarrier wave and the selection of the lower modulation sideband thereof,the subband A would then occupy the portion 3.0-2.1 kilocycles of thisfrequency band, subband B the portion 2.1-1.2 kilocycles thereof, andsubband C the portion 1.2-.3 kilocycles thereof. However, this has notbeen done as only the subband A has been so shifted, but in doing so itwill be apparent that the frequency components of the lower sideband ofthe carrier wave developed in the output circuit of modulator I 9, underthe actual conditions assumed, correspond to the subband A shifted inthe frequency spectrum to the relative position normally' occupied bythe subband B. The term "relative position,'as here used, refers to therelated positions of the subbands as they occur in the speech signal Y.v Similarly, the lower sideband of the modulated carrier wave developedin the output circuit of modulator 20 includes frequency components inthe band .3-1.2 kilocycles in which the frequencies .3 and 1.2kilocycles correspond, respectively, to the frequencies 7.8-6.9kilocycles of the subband B of speech signal Y. The frequency componentsof this lower sideband thus correspond to thesubband B inverted assubband E and shifted into the relative position normally occupied bysubband C. Lastly, the lower sideband of the modulated carrier wavedeveloped in the output circuit of the modulator 2| includes frequencycomponents in the band 3.0-2.1v kilocycles in which the frequencies 3.0and 2.1 kilocycles correspond, respectively. to the frequencies 6.9 and6.0 of the subband C of speech signal Y. The

frequency components of the latter sideband thus correspond to thesubband C shifted to the relative position normally occupied by thesubband A. Thus, it will be evident that when these lower modulationsidebands are subsequently selected and added, in a manner presently tobe considered, there is derived the ultimate speech signal of modifiedintelligibility comprising subbands arranged in the manner designated ascode No. 7

in the ltabulation of Fig. y3. It can similarly be shown that when thevertical bar 8| of switch 50 is energized by unit 49, the subbands E andC vmodified intelligibility and occupies the relative position normallyoccupied by the subband B. This arrangement of subbands is designated inFig. 3 as code No. 5.

Since the subbands are interchanged at the end of 50millisecondintervals as each of the control circuits of the electronic switch 49becomes successively energized, and in accordance with the particularcode card used in unit 5l, it is possible to combine the subbands inconformance with the tabulations of Fig. 3 in a very large number ofpermutations and combinations. It may here be stated that from suchpossible combinations and permutations, it is possible to useapproximately 500,000 different code cards, each of which defines apreconceived plan of `operation. Each such code card, of course, will beused for adesired interval of a few minutes, an hour, or a day, but ispreferably changed at the end of every few minutes according to aprearranged plan by which a limited number of such code cards, forexample 10, could be utilized over a twenty-four hour operating period.

The modulated carrier waves developed in the output circuits of themodulators I9, 20 and 2| are amplified by the respective amplifiers 22,24l

and 25, added in the common output circuit of the latter units, andtranslated through the lowpass filter 22 which selects the lowersideband p of each, as previously stated, to derive a second speechsignal having frequency components lying only in the frequency band of.33.0 kilocycles as do the frequency components of the original speechsignal X, but the second speech signal, unlike the original speechsignal, has greatly modified intelligibility.v 'I'his signal is .appliedto the modulator of unit 29 to modulate the carrier wavegeneratedtherein. There are also applied to the modulator of this unit,simultaneously to modulate the carrier wave thereof, synchronizingpotential pulses derived from orderat inervals not substantially longerthan the longest syllabic interval of the speech signal to derive anultimate or second signal to be transmitted Vhaving its intelligibilitymodified to lators i9', 20' and 2i are coupled through india highdegree. It will further be evident that this means includes means forderiving a plurality of displacement carrier waves, comprising units 29tov 36, inclusive. It will alsob be evident thatunits 1, t9 and 50comprise means including electronic switching means for successivelyapplying the carrier waves of units 29 to 38, inclusive, individually tothe' modulators i9, 20 and 2l in a predetermined order during intervalsnot substantially longer than the longest syllabic interval of thespeech signal, thus to .derive the speech signal of modifiedntelligibility to be transmitted.

In connection with the electronic switch t9, it will -be seen from thedescription thereof that this switch comprises a plurality ofcascadeconnected vacuum tubes l, 15, 62, 63, etc., the

first and alternate ones of the'tubes, for example` tubes 1B, 62, etc.,in the cascade arrangement normally having high conductance and theother of the tubes normally having substantially zero conductance. Theswitch also includes means for coupling the tubes in pairs to cause thetubes successively in pairs to traverse a complete cycle of change ofconductance in response to the application of a negative controlpotential or control signal to lthe first tube. This meansr com'- prisesthe circuit elements 6G, 65, 66, 6l in'units 5| to 58, inclusive,elements 1S. l1, 'i8 and 'i9 in unit 10, and the couplingcondensers 69.The electronic switch further includes a plurality of control circuitsindividually so coupled'to the pairs of tubes that each of the controlcircuits is energized during thechange of conductance of one of thetubes ofthe pair associated therewith, such control circuits comprisingcontrol-circuit terminals 6i, S8 of the units 5l to 58, inclusive,whereby the control circuits are successively energized. There isadditionally included in the electronic switch means for coupling theoutput circuit of the last of the tubes in the cascade arrangement tothe input circuit of the first tube thereof to cause the controlcircuits to be successively and repeatedly energized, this meanscomprising the coupling condenser S9'.

Fig. 4 is a circuit diagram, partly schematic, of a completecarrier-wave receiver suitable for use .with the transmitter arrangementof Fig. 1 in'a secrecy communication system embodying the invention.Certain of the elements of this receiver correspond to similar elementsof the transmitter arrangement of Fig. l andare designated by the samereference numerals primed;

' The receiver includes means for receiving the signal of modifiedintelligibility comprising a. carrier-wave receiver Il 'having an inputcircuit connected to an antenna ground system ill, H2. The receiver alsoincludes frequency-responsive means for deriving from the receivedsignal frequency-component subbands thereof corresponding to individualportions of the frequency band of the speech signal toy be reproduced.This means comprises band-pass filters its, il@ and H5 corresponding,respectively, to band-pass filters I 6, i1 and i3 of the transmitterarrangement but having respective pass bands of 2.1-3.0 kilocycles,1.2-2.1 kilocycles, and .31.2 kilocycles. Each of the band-pass filtershas an input circuit which is coupled to the output circuit of thecarrier-wave receiver H0 and each includes an output circuit which iscoupled to a signal input circuit of an individual one of the modulatorsi9', 20 and 2l'. The output circuits of moduband of 6.0-8.7 kilocycies.The output circuit of unit I I8 is coupled to an input circuit of amodudit vidual amplifiers 23', 24' and 25' to the input circuit of aband-passfllter H6 having a pass lator i4', the latter having anadditional input circuit to which is coupled a carrier-wave generatorl5. The output circuit of modulator i4' a pass band of SOO-3,000 cycles,to the inputcir-` cuit of an audio-frequency amplifier H8, and

there is coupled to the output circuit of the-lattery l unit a suitablereproducing device comprising a v loud-speaker HB.

As in the transmitter arrangement of Fig. l, there are coupled tocarrier-wave input circuits of the modulators i9', 20' and l2| acarrier-wave repeater unit 'i'. includes means for separating thesynchronizingsignal components of the received carrier wave, and thesecomponents are applied to the input circuit including input-circuitterminal 'il' of electronic switch t9', there being the difference inthe electronic-switch 69 of the receiver that the output of the lastunit thereof is not coupled to the input circuit of the additional unit,asv in the electronic switch t9 of the transmitter 'ar' rangement: Thisarrangement insures operation of unit 59 only in synchronism with unitd@ of the transmitter. The control circuits of the electronic switch i9are coupled to the vertical Ybars of the manually-operable` switch andthe horizontal bars thereof are coupled to the input electrodes of therelay devices of unit l' as at the transmitter. Also coupled to theinput electrodes of the repeater devices of unit l are a plurality ofdisplacement carrier generators 29' to '36', inclusive, in the samemanner as at the transmitter.

Considering now the operation of the receiver arrangement justdescribed, the carrier wave radiated by the transmitter is received bythe carrier-wave receiver H0 and the modulation com- POnents thereof,comprising the signal of modied intelligibility, are derived in unit H0and applied to the band-pass filters H3, iil and H5. TheA latter unitsderive the frequency-component subbands of the signal of-modiiledintelligibility and apply these subbands to the respective modulatorsi9', 2li and 2l'. The synchronizingsignal components of the receivedcarrier wave accordance with the plan of transmission determined by themanually-operable switch 5t', and the relay devices, upon being thusenergized, op-

lerate to apply such of the carrier waves of units 29 to 3S as arenecessary successively to interchange the frequency-component subbandsderived by units H3, llt and H5 relative to each other at intervalscorresponding to the intervals at which these subbands were interchangedat the transmitter, but in reverse order to that at the transmitter,thus to derive in the output cirl cuits of modulators I9', 20' and 2lmodulated The carrier-Wave receiver H0 bands are amplied by amplifiers28'. 24' and 25 and applied to a speech-signal input circuit of theband-pass filter IIB which selects the proper sidebands and applies themto an input circuit of modulator lI4. There is also applied to'acarrier-wave input circuit of the latter unit a car- `rier wavegenerated by the carrier-wave generator |5, thereby to provide amodulated carrier wave, the lower sideband of which lies in thefrequency band of .3-3.0 kilocycles and correspends to the speech signalto be reproduced, which, of course, is the speech signal X of fullintelligibility derived by the microphone I2 of the transmitter. Thissideband is selected by W-pass illter H1 and applied to theaudio-frequency amplifier H8 where it is amplified and the amplifiedsignal is then applied to the reproducing device ||9 for reproduction inconventional manner.

From the above description of the receiver arrangement of Fig. 4, itwill be evident that units 1', Il', I5', I9', 20', 2|', 29' to 36',inclusive, 49', 50', III and |I1 comprise means for successivelymodifying the frequency-component subbands derived by the band-passfilters H3, H4 and ||5 relative to each other at intervals correspondingto the intervals at which the subbands were interchanged at thetransmitter, but in reverse order to the predetermined order ofmodiiication at the transmitter. and for adding the modified subbands toderive the speech signal to be reproduced. It will further be evidentthat this means includes electronic switching means and also includesthe means for deriving the plurality of disly-tuned band-pass ill'ters|2| to |21, inclusive7 each of which selects and translates onlythefundamental frequency or a desired harmonic-frequency component of thecarrier wave generated by unit |20. The output circuit of each of theband-pass filters |2| to |20, inclusive, iscoupied to the inputelectrodes of the repeater devices of unit 'l inthe same manner as thecarrier-wave generators 29 to 20, inclusive. of the Fig. 1 arrangement.There is the difference in the present arrangement over the use ofthe'carrier-wave generators to 20, inclusive, of Fig. 1, that since thecarrier waves translated by the band-pass filters |2| to |28, inclusive,are harmonically related, lt can be demonstrated that fewer carrierwaves are vrequiredl in the Fig. -5 arrangement than in that of Fig. 1,since a greater number of the repeater devices in unit 1 may properly becoupled to the output circuit of certain ones of the band-pass illters|2| to |20,inclusive, in effecting the derivation of the ultimate speechsignal of modified intelligibility. 'I'he output of the band-pass filter|21 comprises the fundamentalfrequencyl component of the carrier wavegenerated by unit |20 and thus is used as a synchronizing signal whichis appliedto the modulator of unit 20 of the transmitter toV betransmitted simultaneously with the required synchronizing signal forthe electronic switches 40 and 4l'.

16 When the arrangement of Fig. 5 is used at the receiver, unit |21 isdispensed with and the synchronizing-signal componentl of the receivedcarrier wave is applied to a synchronizing circuit of the carrier-wavegenerator |20 in conventional manner to synchronize its operation withthe corresponding unit of the transmitter apparatus. The operation ofthe Fig. 5 arrangement is otherwise essentially similar to' thatdescribed above in connection with Fig. 1 and will not be repeated.

As illustrative of specific frequency values rsuitable for use with themodified form of displacement carrier-wave generating means of the typeshown in Fig. 5, the following values may l be used:

Kilocycles Frequency of carrier-wave generator 2 From the abovedescription o f the invention, it will be evident that asecrecycommunication system embodying the invention provides an exceptionallyhigh degree of unintelligibility of the transmitted speech signal. 'I'heunintelligibility of the signal is due largely to two factors. The firstof these concerns the particular manner, heretofore described, ofinterchanging and inverting the frequency-component subbands of thespeech signal. The second factor'concerns the rate of interruption ofthe speechsignal: that is, the rate at which the frequency-componentsubbands are interchanged and inverted relative to each other. In thearrangement hereinbefore described, such interruption occurs at 50millisecond intervals, which is equivalent to a rate of interruption oftwenty times per second. This rate of interruption is one which has beenfound by actual operation to be quite satisfactory from the standpointof rendering the transmitted speech signal highly unintelligible. Thelower limit at which the rate of interruption is reasonably eiective isperhaps of the order of 'the average word interval which, as haspreviously been pointed out. is approximately one hundred times perminute or an ining bands of interruption rate at which theintelligibility of the speech is found to be satisfactory, that an upperlimit of interruption rate occurs at approximately 2,000 times persecond. and that above this limit the intelligibility of the speech isno longer impaired by interruption.

While the invention has been described as em` bodied in an`arrangementwherein the ultimate signal of modied'intelligibility has frequency`components in the frequency band of .33.0 kiloacoaose cycles, it will beevident that the frequencycomponent subbands A to F, inclusive, of thespeech signal Y may, by proper choice of the carrier-wave generators29-36, inclusive, be so shifted with relation to each other that theultimate signal of modified intelligibility may have frequencycomponents which occupy any deaired band of frequencies located in anydesired.

portion of the frequency spectrum.

While there has been described .what is at present considered to be thepreferred'embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modiiications may be madetherein without departponent subbands corresponding to individuallportions of said frequency band, the frequencycomponent subbands of saidsecond signal being successively modified relative to each other' in apredetermi-ned order at intervals not substantially longer than thelongest syllabic interval of said speech signal, and means fortranslating and receiving said second signal and for deriving therefromsaid first-named signal.

2. A secrecy communication system comprising, an input circuit adaptedto have applied thereto a speech signal to be transmitted, said signalcomprising frequency components extending over a predetermined frequencyband, means for deriving from said signal a second signal comprising aplurality of frequency-component subbands corresponding to individualportions of said frequency band, the frequency-component subbands f saidsecond signal being successively shifted relative to each other in apredetermined order at intervals not substantially longer than thelongest syllabic interval of said vspeech signal, and means fortranslating and receiving said second signal and for deriving therefromsaid first-named signal.

3. A secrecy communication system comprising, an input circuit adaptedto have applied thereto a speech signal to be transmitted, said signalcomprising frequency components extending over a predetermined frequencyband, means for derivingr from said signal a second signal comprising aplurality of frequency-'component subbands corresponding to individualportions of said frequency band,` the frequency-component subbands ofsaid second signal being successively modified relative to each otherand the frequency components of at least one` such subband beinginverted in the frequency spectrum in a predeter` mined order atintervals not substantially longer than the longest syllabic interval ofsaid speech signal, and means for translating and receiving said secondsignal and fir deriving therefrom said first-named signal.

4. A secrecy communication system comprising, an input circuit adaptedto have applied thereto a speech signal to be transmitted, said signalcomprising frequency components. extending over a predeterminedfrequency band, means for deriving from said signal a second signalcomprising a plurality of frequency-component subbands corresponding toindividual portions of said y frequency band, the frequency-componentsubbands of said second signal being successively modified relative toeach other and the frequency components of at least one of such subbandsbeing shifted in the frequencyspectrum in a predetermined order atintervals not substantially longer than the longest syllabic interval ofsaid speech signal, and means for translating and receiving said secondsignal and for deriving therefrom said first-named signal. v

5. A secrecy communication system comprising, an input circuit adaptedto have applied thereto a speech signal to be transmitted, said signalcomprising frequency components extending over a predetermined frequencyband, means for deriving from said signal a second signal comprising a.plurality of frequency-component subbands corresponding to individualportions of said frequency band, the frequency-component subbands ofsaid second signal being successively modified relative to each otherand the frequency.

components of at least one of such subbands being inverted and of atleast another of said subbands being shifted in the .frequency spectrumin a predetermined order at intervals not substantially longer than thelongest syllabic interval of said speech signal, and means fortranslating and receiving said second signal and for deriving therefromsaid first-named signal.

6. A secrecy communication system comprising, an input circuit adaptedto have applied thereto a speech signal to be transmitted, said signalcomprising frequency components extending over a predetermined frequencyband, means for deriving from said signal a second signal comprising a.plurality of frequency-component subbands corresponding to individualportions of said frequency band, the frequency-component subbands ofsaid second signal being successively modified relative to each other`and thev frequency components of at least one of Vsuch subbands beinginverted and shifted in the frequency spectrum in a predetermined orderat intervals not substantially longer than the longest syllabic intervalof said speech signal, and means for translating and receiving saidsecond signal and for `deriving therefrom said first-named signal.

7. A secrecy communication system comprising, an input circuit adaptedto have applied thereto a, speech signal to be transmitted, said signalcomprising frequency components extending over a predetermined frequencyband, electronic switching means for deriving from said signal a secondsignal comprising a plurality of frequency-component subbandscorresponding to individual portions of said frequency band, thefrequency-'component subbands of said second signal being successivelymodified relative to each other in a predetermined order at intervalsnot substantially longer than the longest syllabic interval of saidspeech signal, and means for g translating and receiving said secondsignal' and for derivingi'rom said signal a second signal comprising aplurality of frequency-component subbands corresponding to individualportions of said frequency band, the frequency-component subbands ofsaid second 'signal being successively 19 modified relative to eachother in a predetermined order at intervals of substantially fiftymilliseconds durationland means for translating and receiving saidsecond signal and for deriving therefrom said first-named signal.

9. A secrecy communication system comprising, an input circuit adaptedto have applied thereto a speech signal to be transmitted, said signalcomprising frequency components extending over a predetermined frequencyband, means for deriving from said signal a second signal comprising aplurality of frequency-component subbands corresponding to individualportions of said. frequency band, the frequency-component sub-bands'ofsaid second signal being successively modified relative to each other ina predetermined order at intervals not substantially longer than thelongest syllabic interval of said speech signal, manually-adjustableswitch means forsaid frequency band, the' frequency-component subbandsof said second Signal being successively modified relative to eachotherin a predeterl mined order at intervals not substantially longer saidfrequency band, 'the frequency-component subbands of said second signalbeing successively modified relative to each other and the frequencycomponents of at least one of such subbands being shifted in thefrequency spectrum in a predetermined order at intervals notsubstantially longer than the longest syllable interval of said speechsignal, and an output circuit .coupled to said means to have appliedthereto said second signal, whereby the signal applied to said outputcircuit has modified intelligibiiity and is suitable for transmissiontherefrom to a remote point while maintaining a high degree of secrecyloi! communication.

13. A secrecy communication transmitter comprising. aninput circuitadapted to have applied thereto a speech signal to betransmittedsaidsignal comprisingfrequency components extending over a predeterminedfrequency band, means for deriving from said signal a second signalcomprising a plurality' of vfrequency-component subbands correspondingto individual portions of said frequency band. the frequency-componentsubbands of said second signal being successively modified relative toeach other and the frequency components of at least one of such subiandsbeing inverted and of at least another of such subbands being shifted inthe frequency spectrum in a predeterminedvorder at intervals notsubstantially longer than the longest syllable interval ofv said speechsignal, and an output circuit coupled to said means' to have appliedthereto said second Isignal, whereby the signal applied to said outthanthe longest syllable interval of said speech i signal, and anA outputcircuit coupled to said means to have applied thereto said secondsignal, whereby the signal applied to said output circuit has modifiedintelligibility and is suitable for transmission therefrom to a remotepoint while maintaining a high degree of secrecy ofcommunication.

11. A secrecy communication transmitter comprising. an input circuitadapted to have applied thereto a speech signal to be transmitted, saidsignal comprising frequency. components extemling over a predeterminedfrequency band, means for deriving from said signal a second signalcomprising a plurality of frequency-component sub- Y bands correspondingto individual portions otiaid frequency band, the frequency-componentsubbands o f .said second signal being successively modified relative toeach other and the frequency components of atleast one of such subbandsbe- .inginvertedinthefrequencyspectnnnina'predetermined order atintervals not substantially longer than the longest syllable interval ofsaid lspeech signal, and anoutput circuit coupledto said means to have`applied thereto said second signal, whereby the signal applied to saidoutput circuit has modified intelligibility and is suitable for ontherefrom to a remote pointwhilemaintainingahighdegreeofsecrecy ofcommunication.

put circuit has modified intelligibility and is suitable fortransmission therefrom to a remote point while vmaintaining a highdegree of secrecy of communication. Y

i4. A secrecy .communication transmittercomprising, an input circuitadapted to have applied thereto a speech signal to be transmitted, saidsignal comprising frequency components extending over a predeterminedfrequency band, electronic switching means for deriving from said signala second signal comprising a plurality of frequency-component subbandscorresponding to individual portions of said frequency band, thefrequency-component subbands of said second signal being successivelymodiiied relativeto each other in a predetermined order at intervals notsubstantially longer than the longest syllable inferval of said speechsignal. and an output circuit coupled to said means to have appliedthereto said second signal, whereby the signal applied to said outputcircuit has modified intelligibility and is suitable for tron therefromto a remote point while maintaining a high degree of secrecy ofcommunication.

15. VA secrecy communication transmitter comprising, an input circuitadapted to have applied thereto a speech sign'al to be transmitted, saidsignal comprising frequency components extending vover a predeterminedfrequency band. means for deriving from said signal a second signalcomprising a plurality of frequency-component subv bands correspondingto individual portions of said 12. A secrecy communication transmittercomprising, an input circuit adapted to have appliedtheretoaspeechsignaliobetransmitted.said signal comprising frequencycomponent extending over a predetermined frequency band, means forderiving from said signal a second signal comprislng a plurality offrequency-component subbands corresponding to individual portions of.

frequency band, the frequency-component subbands of said second signalbeing successively modined relative to eachother in a predeterminedorder at intervals of substantially ility milliseconds duration, and anoutput circuit coupled to said means to have applied thereto said secondsignal, whereby the signal applied to said output circuit has modiiiedintelligibility and is suitable for transmission therefrom to a remote`.2l point whilemaintaining a high degree of secrecy of communication.

16. A secrecy communication transmitter comprising, an input circuitadapted to have applied thereto a, speech signal to be transmitted, saidsignal comprising frequency components extend- \\ing over a,predetermined frequency band, means,

ing and modifying as desired from time to time y the said predeterminedorder of interchangingof said frequency-component subbands, and anoutput circuit coupled to said means to have applied thereto said secondsignal, whereby the signal applied to said output circuit has modifiedintelligibility and is suitable for transmission therefrom to a remotepoint while maintaining a high degree of secrecy of communication.

17. A secrecy communication transmitter comprising, an input circuitadapted to have applied thereto a speech signal to be transmitted, saidsignal comprising frequency components extending over'a predeterminedfrequency band, means for separating said speech signal into a pluralityof frequency-component subbands corresponding to individual portions ofsaid frequency band,

means for successively modifying said subbands 35 tive to render saidone repeater devices operarelative to Veach other in a predeterminedorder at intervals not substantially longer than the longest syllabicinterval of said speech signal to derive a second signal having modifiedintelligibility, an` output circuit coupled to said last-named means tohave applied thereto said second signal, whereby the signal applied tosaid output circuit is suitable for transmission therefrom to a re-'mote point while maintaining a highdegree of secrecy of communication. Y

18. A secrecy communication transmitter comprising, an input circuitadapted to have applied thereto a speech signal to be transmitted, saidsignal comprising frequency components extending over a predeterminedfrequency band, means for separating said speech signal into apluralityof frequency-component subbands corresponding to individualportions of said frequency band, means for deriving a'plurality ofdisplacement carrier waves, means including said last-named means forsuccessively modifying said subbands relative to each other in apredetermined order at intervals not substantially longer than thelongest syllabic interval of said speech signal to derive a secondsignal having modified intelligibility, and an output circuit coupled tosaid last-named means to .have applied thereto saidr second signal,whereby the signal applied to said `output circuit is suitable fortransmission therefrom to a remote point while maintaining a high degreeof secrecy of communication. l

19. A secrecy communication transmitter comprising, an input circuitadapted to have applied thereto a speech signal to be transmitted, saidsignal comprisingffrequency components extend*- ing over a predeterminedfrequency band, a p lurality of band-pass filters for separating saidspeech signal into a plurality of subbands corresponding to individualportions of said frequency band, a plurality of modulators individuallycoupled to said band-pass filters, means for deriving leficaces;

22 a plurality of displacementfcarrier waves, means including electronicswitching means for successively applying said carrier wavesindividually to saidmodulators in a predetermined order during intervalsnot substantially longer than the longest syllabic interval of saidspeech signal to derive a second signal having modified intelligibility,and

- an output circuit coupled to said modulators to have applied theretosaid'second signal, whereby l0 the signal applied to said output circuitis suitable for transmission therefrom to a remote point whilemaintaining a high degree of secrecy of' communication. Y

20. A secrecy communication transmitter com- -prising, an input circuitadapted to have applied thereto a speech signal to be transmrtted, saidsignal comprising frequency components extending over a predeterminedfrequency band, a plurality 'of band-pass filtersv for separating 'saidspeech signal into a plurality of frequency-component subbandscorresponding to individual portions of said frequency band, a pluralityof modulators individually coupled to said filters, means for deriving aplurality of displacement carrier 5-waves, a plurality Iof repeaterdevices arranged in groups each adapted to apply a predetermined groupof said carrier waves to individual ones of said modulators, electronicswitching means having a, plurality of control circuits successively oenergized at intervals not substantially longer than the longestsyllabic interval of said speech signal, each of said control circuitsbeing coupled to one repeater device in each of said groups of repeaterdevices and when energized being effeccessively to modify saidfrequency-componentV subbands relative to each other at said intervals,

40 and an output circuit coupled to all of said modulators for combiningthe interchanged frequencycomponent subbands to/ derive a second signalhaving modified intelligibility, whereby the signal derived in saidoutput circuit is suitable for transmission therefrom to a remote pointwhile maintaining a high degreeof secrecy of communication.

21. A secrecy communication transmitter comprising, an input circuitadapted to have applied thereto a speech -signal to be transmitted, saidsignal comprising frequency components extending over a predeterminedfrequency band, a plurality of band-pass filters for separating saidspeech signal into a plurality offrequency-component subbandscorresponding to individual portions of said frequency band, a pluralityof modulators individually coupled to said filters, means for deriving aplurality of displacement carrier Waves, a. plurality of repeaterdevices arranged in groups each adapted to apply a predetermined groupof said carrier waves to individual ones of said modulators, electronicswitching means having 'a plurality of control circuits successivelyenergized at intervals not -substantially longer than the longestsyllabic interval of said speech signal, each o1 said control circuitsbeing coupled to one repeater device in each of said groups of repeaterdevices and when energized being effective to render said one repeaterdevices operative to'apply a group of said carrier waves to individualones of said modulators, thereby successively to modify saidfrequency-component subbands relative to each other at said intervals,manually-adjustable switching means for establishingand modifying asdesired from time to amaca 'component subbands corresponding toindividual portions of the frequency band of said speech 24 l liedrelative to eachother in a predetermined orderV at intervals notsubstantially longer than the longest syllabic interval of said speechsignal,

a plurality of band-pass filters for deriving said frequency-componentsubbands of said received Signaka plurality of modulators individuallycoupled to said'iilters, means for deriving a plurality of displacementcarrier waves, and means including electronic switching means forsuccessively signal but4 having said subbands successively modiiledrelative to each other in a predetermined order at intervals notsubstantially longer than the longest syllabic interval of said speechsignal, frequency-responsive means for deriving said frequency-componentsubbands of said received signal, and means for successively modifyingsaid derived frequency-component subbands Arelative to eachother atintervals corresponding to said nrst-nam'ed intervals but in reverseorder to said predetermined order and for adding said last-named modiedsubbands to derive said Silmal to be reproduced. l

23. A secrecy communication receiver comprising, means for receiving asignal of modified intelligibility which is` derived from a speechsignal to be reproduced and which includes frequencycomponent subbandscorresponding to individual portions of the frequency band of saidspeech signal but having said subbands successively modiiled relative toeach other in a predetermined order at intervals not substantiallylonger than the longest syllabic interval oi said speech signal,frequency-responsive means forderiving said frequency-component subbandsof said received signal, and electronic switching means for successivelymodifying said derived frequency-component subbands relative to eachother at intervals corresponding to said rst-named intervals but inreverse order to said predetermined order and for adding said last-namedmodiiied subbands to derive said signal to-.be reproduced.

24. A secrecy communication receiver comprising, means for receiving asignal of modified intelligibility which is derived from a speech signalto be reproduced and which includes frequencycomponent subbandscorresponding to individual portions of the frequency band of saidspeech signal but having said subbands successively modled relative toeach other in a predetermined orderat intervals not substantially longerthan the longest syllabic interval vof said speech signal,frequency-responsive means for deriving said frequency-componentsubbands ot said received signal, means for deriving a plurality ofdisplacement carrier waves, and means including said last-named meansfor successively modifying said 'derived frequency-component subbands`relative applying said carrier waves individually to said modulators ina predetermined order during intervals corresponding to said Brat-namedintervals successively to modify said derived frequency-componentsubbands in reverse` order to said predetermined order and for addingthe outputs of said modulators. to derive said signal to be reproduced.

28. An electronic switch comprising, a plurality of cascade-connectedvacuum tubes, means inter.. coupling said tubesin pairs to causesaidtubes successively in pairs to traverse a complete cycle of changeof conductance in response to the application of a control signal to theilrst of said cascaded tubes, and a plurality of control circuitsindividually so coupled to said pairs of tubes thateach of said controlcircuits is energized during the change of conductance of one of thetubes of the pair associated therewith, whereby said control circuitsare successively energized.

27. An electronic switch comprising, a plurality of cascade-connected-vacuum tubes, alternate ones of said tubes in the cascade arrangementnormally having high conductance and the other of said tubes normallyhaving low conductance, means intercoupling said tubes in pairs to causesaid tubes successively in pairs to traverse a com.. plete cycle ofchange of conductance in response to the application of a control signalto the first oi' said cascaded tubes, and a plurality of controlcircuits individually so coupled to said pairs oi' tubes that each ofsaid control circuits is energized during the change of conductance ofone of the tubes of the pair associated therewith, whereby said controlcircuits are successively energized.

28. An electronic switch comprising, a plurality of cascade-connectedvacuum tubes, a ilrst and alternate ones of saidv tubes in the cascadearrangement normally having high conductance and the other of said tubesnormally having low conductance, means intercoupling said tubes in pairswith a normally high conductance tube as the iirst tube of each pair tocause said tubes successively in pairs to traverse a complete cycle ofchange of conductance in response to the application of a control signalto the ilrst oi' said cascade tubes, and a plurality of control circuitsindividually so coupled to said pairs oi tubes that each of said controlcircuits is energized during the change of conductance of one of thetubes of the pair associated therewith, whereby said control circuitsare successively energized.

29. An electronic switch comprising, a plurality Aof cascade-connectedvacuum tubes, means 5 dntercoupling said'tubes in pairs to cause saidcomponentsubbands corresponding to individual portions of the frequencyband of said speech sig- .1 hm. having said subbands successivelymoditubes'successively in pairs and at the ends of predeterminedequaltimeintervalstotraverseacomplete cycle of change of conductance inresponse tothe application of acontrol signal to the iirst i of saidcascade tubes, and a plurality of cmtrol circuits individually soVcoupled to said pairs of 30. An electronic switch comprising, aplurality of cascade-connected vacuum tubes, means intercoupling saidtubes in pairs to cause said tubes successively in pairs to traverse a,complete cycle of change of conductance in response to the applicationof a control signal to the first of said cascaded tubes, a plurality ofcontrol circuits ineach of said control circuits ls energized during thechange of conductance of one of the tubes of the pair associatedtherewith, and means cou pling the output circuit of the last ofsaid/cascade tubes to the input circuit of the first tube thereof,whereby said` control circuits are successively and repeatedlyenergized.

31. An electronic switch comprising, a plurality of cascade-connectedvacuum tubes, the first and y dividually so coupled to said pairs oftubes that accanto 26 alternate ones of said tubes in the cascadearrangement normally having high conductance and the other of said tubesnormally having low conductance, means intercoupling said tubes in pairswith a normally high conductance tube as the first tube of each pair tocause said tubes successively in pairs to traverse a complete cycle ofchange in conductance in response to the application of a negativepotential pulse to the first of said cascade tubes, anda plurality ofcontrol circuits individually so coupled to said pairs of K sized duringthe change of 'conductance of one X tubes that each of said controlcircuits is enerof the tubes of the .pair associated therewith; wherebysaid control circuits are successively s energized.

JAMES F. CRAAIB.

